Established one week after the 2011 Fukushima disaster, the organisation says it has collected 15 million data points with the portable counters and about 320 fixed censors installed across the country.

About two-thirds of the data comes from Fukushima. It has been published on the group's website - blog.safecast.org - and also converted into maps.

"Why do the people need our help? Maybe they trust us more than the government," says Azby Brown, a Safecast researcher who lives in Japan. "We are independent, and we guard our independence very rigidly."

The organisation has more than 100 regular volunteers, including engineers, radiation experts and hardware designers across the country.

Advisers include tech entrepreneurs and Jun Murai, a computer scientist and dean at Keio University who was recently recognised for the early design and development of the internet.

Kiyoshi Kurokawa, a physician who led the Japanese parliament's independent investigatory commission into the Fukushima nuclear crisis, says the work of Safecast's volunteers is credible. "They are well-educated and careful and very scientific in their work," he says.

The disaster, followed by a string of mishaps and withheld information, has dampened public trust in the national government and Tokyo Electric Power Company, which operates the Fukushima plant. Both have been criticised for delays and poor oversight, worsening the nuclear disaster.

Many residents near the power plant have complained that they weren't evacuated until after hours of delay.

The government now discloses radiation levels on its website. But many people have said that they usually disregard the official data.

Brown, from Safecast, says the government hasn't established enough measurement data points to give people a comprehensive picture of the contamination levels. In one neighbourhood in Fukushima's Koriyama City, the government's main radiation information web page shows data from just seven monitoring posts, while Safecast's map of the same area shows about 800 data points.

His organisation has teamed up with local governments in three Fukushima cities to launch a programme called "Street-by-street". Each town has been loaned about 10 Geiger counters from Safecast. These devices are attached to the local post office's motorcycles to collect data around the towns.

"Our condition is that the data has to be open," Brown says. "Everybody must be able to use it."

The programme allows local officials to circumvent restrictions imposed by the national government on data collection.

"Local governments are happy to have an alternative source of information to share with their community," Brown says.

This article appeared in the South China Morning Post print edition as: